1. Field of the Invention
This invention relates to a method of producing a high-density of wiring board for mounting electronic devices, and more particularly to a method of producing a wiring board having the terminal parts of a wiring pattern for connection to the input and output terminals of electronic devices to be mounted thereon.
2. Description of the Related Art
The mounting circuit devices which have various electronic devices such as a semiconductor element mounted on a wiring board have been finding extensive utility in circuit components of electronic devices. The construction of mounting circuit devices of this kind demands densification or compaction of circuits. For the sake of meeting this demand, the so-called multilayer type wiring boards have come to be used. More specifically, the electronic circuits such as of a computer, for example, have come to be provided with high-density packaging means capable of shortening wirings for the purpose of allowing quick propagation of signals among semiconductor elements. In short, the demand for densification of circuits is met by mounting semiconductor elements in the form of bare chips thereby reducing the relevant areas necessary for mounting and using wiring boards of a small wiring width thereby increasing the packaging density and, at the same time, reducing the length of wirings.
Incidentally, the multilayer type wiring board mentioned above is generally manufactured by means of following procedures. Wiring patterns are formed severally on copper foils which are applied one each to the opposite surfaces of an insulating substrate, then other copper foils are superposed in place through an insulating sheet (such as, for example, a prepreg layer) on the surfaces now bearing the wiring patterns, and all the superposed layers are laminated into one piece by simultaneous application of heat and pressure. Then, the produced laminate is perforated as prescribed by means of drilling, for example, and the inner wall surfaces defining the holes are coated with a metallic layer by the chemical plating technique and the metallic layers in the holes are thickened by electroplating to effect electrical connection between the wiring layers of the inner-layer wiring pattern and the outer-layer wiring pattern. Thereafter, wiring patterns including terminal parts (connecting pads) are formed on the surface copper foils to complete a multilayer wiring board. When the multilayer type wiring board happens to be using wiring pattern layers copiously, it is manufactured by a method which resides in allowing addition to the number of double-faced type wiring boards to be interposed between the opposite surface wiring boards.
As a multilayer type wiring board which is different in construction from that described above, a wiring board which is manufactured by first superposing thin film multilayer wiring layers using an intervening insulating layer of a polyimide type resin one each on the opposite surfaces of a wiring substrate using an intervening insulating layer of a ceramic substance and then laminating the superposed layers into one piece has been proposed for mounting. When semiconductor elements (LSI) are mounted in their original form of bare chips on the multilayer type printed wiring board manufactured by the procedure described above for the purpose of attaining high-density packaging, the bare chips are provided on the input and output terminal sides thereof with bumps of solder and are connected through the medium of these solder bumps to the terminal parts (pads) on the surface of the multilayer type printed wiring board.
The means for mounting the bare chips through the medium of the solder bumps mentioned above, however, demands that the solder bumps have a minute size conforming to the size of the input and output terminals of the bare chips being mounted and, therefore, is at a disadvantage in not only complicating the formation of the solder bumps but also imposing a handicap on the yield on account of the shape, size, position, and accuracy of the solder bumps. Meanwhile, the wiring board, for the sake of meeting the demand for densification of packaging, requires not only the wiring width to be decreased but also the terminal parts (pads) for connection of the input and output terminals of the bare chips to be miniaturized or the intervals between the terminal parts (pads) to be narrowed.
Further, in this multilayer type wiring board, the through holes interconnecting the wiring patterns have their diameter automatically limited in the light of the improvement of the wiring density mentioned above and the improvement of packaging density. When the through holes have their diameter set in the neighborhood of 0.01 mms, for example, since the manufacture of these through holes defies effective use of an NC drill machine, a new machining device or machining means must be prepared for the perforation.
In any event, the multilayer type wiring board of the construction heretofore known in the art incurs various problems from the viewpoint of densification and compaction of mounting circuit devices.